This study aimed to determine the effect of the implementation of the Contextual Teaching and Learning (CTL) model with the outdoor approach towards students' ability in mathematical representation. It was quasiexperimental research consisting of two experimental classes and one control group. It used a pretest-posttest control group design. The population of this study was the students of SUPM Tegal. Sampling was conducted using cluster random consisting of three classes. The first experimental group was carried out by implementing the learning model of CTL with the outdoor approach. The second experimental group was carried out by implementing a learning model of CTL, while the control group was conducted by implementing a conventional learning model. The research instrument was a 7-point mathematical representation test in the form of an essay. The results of the research were 1) the learning model of CTL with an outdoor approach affected the improvement and achievement of the students' ability in mathematical representation and was higher than CTL and a conventional learning; 2) the improvement of the students' ability of representation in the groups of CTL with outdoor approach, CTL, and conventional learning respectively was in high, medium, and low category.